1. Field of the Invention
The present invention relates to the field of the non-volatile memories and, more specifically, to the management of protection information in non-volatile memory devices of flash type.
2. Description of the Related Art
Flash memory devices are non-volatile memories wherein each cell can be programmed electrically, but a large number of cells, which form a sector, have to be erased at the same time. This allows obtaining a very simple structure of the memory device, which can be manufactured at low cost and with a high density thereby being well suited to several applications.
Generally, a supplementary sector is provided inside the memory device for storing protection information of the (actual) sectors. A reading operation can be performed on each sector; however, an erasing operation or a programming operation is allowed on non-protected sectors only. This mechanism preserves the memory device from undesired erasing and/or programming operations, thereby avoiding any loss of data stored therein.
For example, those memory devices are used in digital systems such as the set-top boxes for DTT (acronym for Digital Terrestrial Television) applications; in such case, the protection information allows stopping the access to the sectors that are used for storing configuration information, such as the identification codes and the activation codes of the different services.
The protection information is updated by overwriting the corresponding sector with a new value. This operation implies the saving of the protection information in a volatile buffer memory, and thus the erasing of the whole protection sector; subsequently, the memory cells of the protection sector can be programmed for obtaining the desired value of the protection information.
A drawback of the above-described solution is that each update of the protection sector needs both an erasing operation and a programming operation. Thus, the updating operation is relatively slow.
Moreover, during each erasing and programming operation the memory cells of the protection sector are subjected to an electric stress; after repeated updating cycles this can cause the loss of the information stored in the protection sector.
In addition, a possible supply voltage drop during the erasing phase of the protection sector could leave it in an erroneous configuration (for example, so as all the sectors of the memory are non-protected from updating operations).